The present invention relates to a technique for injecting fuel by opening and closing a fuel supply path formed between a valve element and its valve seat in a fuel injector, the valve seat being driven by the application of an electrical current to coils of the fuel injector.
In an electromagnetic fuel injector (hereafter simply referred to as an injector), a plunger to which a valve element is attached is withdrawn from a valve seat by an electromagnetic force (electromagnetic attraction force) generated by a coil provided in the injector, in which current flows, whereby fuel is injected. When the electrical current flowing in the coil is stopped, the electromagnetic attraction force decays, and the plunger is pressed back by the force of a return spring in the valve closing direction. Thus, the valve of the injector is closed. In an injector of the above-mentioned type, the valve is required to immediately respond to an opening demand or a closing demand without a time delay in order to attain a wide dynamic range of fuel injection. The dynamic range refers to a range in which a linear relationship exists between the fuel injection amount and the valve opening time width, and is expressed by the ratio of the maximum injection amount to the minimum injection amount.
Conventionally, in order to improve the rise time characteristics of the valve opening operation, the following method has been adopted. That is, a high voltage is generated by providing a voltage step-up circuit, and a large current is caused to flow in an injector coil for a short time by applying the generated high voltage to the coil. For example, Japanese Patent Application Laid-Open 241137/1994 discloses a fuel injection control device in which a voltage step-up circuit is provided in a drive circuit for driving an electromagnetic fuel injection valve, and a voltage of 70 V, which is obtained by boosting a voltage of 12 V obtained from an external power source, using the provided voltage step-up circuit, is applied to a drive coil of the electromagnetic fuel injection valve.
In the above fuel injection control device, the excitation current for the drive coil is controlled so that a target value of the excitation current is set as a high value at an initial valve opening time in which a valve element is operated from a closed valve state to a valve opening state (early period in valve opening and during a process of opening the valve), and a low target value of the current is realized by on/off controlling of the drive coil during a valve open hold period in which the valve element is held at in the open state. Thus, the valve opening response is improved by controlling the excitation current for the drive coil at a high target value, and by controlling the excitation current at a low target value during the valve open hold state. In this way, the wasting of power is avoided, and heat generation is suppressed.
Japanese Patent Application Laid-Open 326620/1996 discloses an electromagnetic fuel injection valve in which two coils A and B are provided, and current is caused to flow in the two coils A and B for a preset period after the start of current flow in the coils during valve opening operations. Further, after a preset period, current flowing in the coil A is stopped, and current flows in only the coil B. In the above electromagnetic fuel injection valve, by causing current to flow in both of the two coils A and B for a preset period after the start of current flow in the coils, a strong magnetic flux can be generated and quick valve opening operations can be performed. Further, since a valve element can be held in an open valve state by a necessary and minimum force produced by only one of the two coils during the valve open hold period, a quick valve closing operation can be performed. Moreover, since a large current flows in the coils only at the time of valve opening, heat generation in the injection valve can be suppressed.
Furthermore, in the fuel injection control device disclosed in Japanese Patent Application Laid-Open 241137/1994, a detector for detecting the fuel feeding pressure (fuel pressure) is provided, and a high target value of the excitation current, or the control period for which the excitation current flows at the high target value, is adjusted, based on the fuel pressure defected by the detector. Thus, deterioration in the injection performance of the electromagnetic fuel injection valve, due to changes in the fuel pressure, is avoided.
In the fuel injection control device disclosed in Japanese Patent Application Laid-Open 241137/1994, in which only one coil is provided in the fuel injection valve, the valve element is controlled by the one coil from the start of valve open operations to the end of valve opening operations (valve closing) through holding of a valve open state.
It is necessary to decrease the current flowing in a coil in order to reduce heat generation or power consumption in the fuel injection valve. However, to obtain a sufficient magnetomotive force for holding a valve open state with a small coil current, it is necessary to increase the number of coil turns. On the other hand, since the rise time of the coil current should be made small to improve the response in valve opening, a greater increase in the voltage applied to the coil is required as the number of coil turns is increased. That is, the fuel injection control apparatus disclosed in Japanese Patent Application Laid-Open 241137/1994 has a structure which contradictory has characteristics relative to attaining both a quick response in valve opening and a low power consumption for the valve open hold period, if the same coil is controlled.
Further, since the above-mentioned voltage set-up circuit is expensive, and since insulation measures for the high voltage are necessary, the production cost is increased by adopting such a voltage step-up circuit. Therefore, in order to reduce the production cost, it is desirable to operate an injector with a lower voltage, and it is even more desirable to operate an injector with a battery voltage of 12 V, thereby eliminating a need for a voltage step-up circuit, if possible. Moreover, if an injector is driven by a lower voltage, fewer measures for securing its safety are required, and the maintenance or the adjustment of the injector becomes easier.
In the electromagnetic fuel injection valve disclosed in Japanese Patent Application 326620/1996, the structure and electromagnetic characteristics for each of the coils A and B are not disclosed. In providing two coils, securing a high response in the valve opening operation impedes the objective of holding a necessary and minimum magnetomotive force, and stably holding a necessary and minimum magnetomotive force causes a limitation on the attainment of a high response during the valve opening operation. Therefore, in accordance with this disclosed arrangement of two coils, it is difficult to attain a quick response of the valve opening operation, that is, largely to increase the valve element attraction force, which will be required in the future.